The invention described herein was made in the performance of official duties by an employee of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon.
The invention relates generally to magnetic sensing systems, and more particularly to a magnetic anomaly sensing array and processing system that uses a plurality of precision synchronized transceivers to directly measure magnetic field strength at a plurality of locations and that processes the measurements to generate quantities indicative of presence, location and classification of a target.
The basic construction of a prior art eddy-current-based active magnetic anomaly sensor system includes a transmitter and a receiver. The transmitter induces anomalous magnetic induction fields in an electrically conductive or magnetic target located in the sensor detection space. The receiver detects/discriminates the anomalous magnetic induction fields propagating from the target. The transmitter typically consists of electronic circuitry that drives a time dependent electrical current through an induction coil to generate a time and vector distance dependent magnetic induction field. The induction coil can be driven by a continuous wave or pulsed signal. When the generated magnetic induction field interacts with a target, anomalous magnetic moments are induced with in the target which, in turn, cause anomalous magnetic fields to propagate from the target. The sensor system""s receiver typically consists of an induction or xe2x80x9csearch coilxe2x80x9d sensor coupled to signal amplification and processing circuitry to condition and process the magnetic fields detected by the search coil. Through Faraday induction, the search coil generates a voltage proportional to the time derivative of the target""s magnetic anomaly fields lying along the search coil""s axis. Such sensor systems have a variety of shortcomings.
For portable active sensor systems having a transmitter and receiver in close proximity to one another, the spatial variation between the actively induced magnetic anomaly field and its time derivative at the receiver decreases with the inverse 6-th power of target-to-receiver distance. Accordingly, to double the detection range of a sensor system, transmitter amplitude or receiver sensitivity must be increased by a large factor, i.e., a factor of 26 or 64. Also, relying on the time derivative of the magnetic anomaly field limits the sensor""s time discrimination capability, receiver bandwidth and low frequency sensitivity.
Another shortcoming of prior art active magnetic anomaly sensing systems is the interference generated by the transmitter at the receiver. Since the transmitter drive fields are many orders of magnitude larger than the target""s induced magnetic anomaly fields, the transmitted signal has a tendency to overwhelm or jam the reception of the much smaller magnetic anomaly fields. Even with specialized transmitter-receiver geometries, systems that use a continuous wave transmitter drive signal tend to lose much of a target""s transient response. To combat this problem, pulsed transmitters are used and operate on the theory that reception occurs when the transmitter is off. While this works to a certain degree, time constant or transient effects of a typical transmitter coil last for tens of microseconds. Unfortunately, it is in this time frame that the strongest target-signature-related magnetic anomaly field transients are generated by the target. Thus, even though the transmitter coil is deactivated, coil transients tend to jam reception of the strongest magnetic anomaly fields. This problem precludes the use of prior art active magnetic anomaly sensing systems in the detection of non-conductive plastic mines in a conductive media (e.g., seawater) since plastic mines have an extremely short transient response.
Still another shortcoming of prior art active magnetic anomaly sensor systems stems from the use of inductive search coils as the magnetic anomaly field sensing element. Specifically, this type of sensing element responds primarily to the time derivative of magnetic flux change components that are parallel to the coil""s axis. Therefore, the sensing element has limited spatial direction sensing capabilities for resolving the direction and magnitude of the three-dimensional vector components that comprise the magnetic anomaly field caused by the target. The lack of three-dimensional resolution limits the system""s target localization and classification capabilities.
To address these shortcomings, U.S. Pat. No. 6,362,625 teaches a magnetic anomaly sensing system having a precision synchronized transceiver that directly measures magnetic field strength for improved detection and/or discrimination of targets. A transmitter is activated and subsequently deactivated. During transmitter activation, a magnetic field is transmitted towards a target such that the magnetic field induces magnetic moments in the target which cause a magnetic anomaly field to propagate from the target. A first multi-axis magnetic sensor is positioned a distance D from the target, while a second multi-axis magnetic (reference) sensor is positioned a distance (D+d) from the target. Outputs from the two sensors are read during the times that the transmitter is deactivated. The second (reference) sensor""s output is subtracted from the first sensor""s output to generate a differential output indicative of the magnetic anomaly field propagating from the target. Means and methods are provided to synchronize the response characteristics of the sensors with one another, and to synchronize the transmitter with the sensors so that deactivation of the transmitter results in a near instantaneous detection of magnetic field transients by the first sensor.
The system disclosed in U.S. Pat. No. 6,362,625 provides for target stimulation by a single transmitter and for the detection and discrimination of correlated three-dimensional information content of target signals at a single point in space. However, this single transceiver configuration still results in ambiguities with regard to the actual location and classification of the target. Also, the effective detection range of the single transceiver configuration is still limited by the aforementioned variation of signal amplitude with the inverse sixth power of target-to-receiver distance. While such target localization and classification ambiguities can be resolved by sweeping the target search area with the single transceiver, this procedure may not always be practical or efficient. For example, robotic platforms used in the localization and classification of targets in a minefield must be able to carry out their target sensing tasks quickly and efficiently as the robotic platform moves through the minefield.
Accordingly, it is an object of the present invention to provide a system that can be used to aid in the detection, localization and classification of magnetic anomalies associated with a target.
Another object of the present invention is to provide a sensing and processing system that can be used to aid in the detection, localization and classification of magnetically polarizable targets.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, an active magnetic anomaly sensing and processing system/method are provided. A plurality of transceivers are positioned at known relative positions on a platform. Each transceiver includes i) a transmitter for transmitting a magnetic field towards a target area to induce magnetic moments in a target which, in turn, cause a magnetic anomaly field to propagate from the target, and ii) a sensor positioned a distance from the target for sensing magnetic field strength and producing a first output indicative thereof. A reference sensor is positioned further from the target than each of the transceivers"" sensors. The reference sensor is capable of sensing magnetic field strength and producing a second output indicative thereof. Each transmitter is selectively activated and subsequently deactivated simultaneously to define a transmission time and a non-transmission time, respectively. Scalar magnitudes of each first output and the second output produced during the non-transmission time form quantities that are indicative of presence, location and classification of the target.